1. Field of the Invention
The present invention generally relates to a pulse generating circuit and, more particularly, to a digital pulse generating circuit for generating a pulse signal having a predetermined time period width based on a trigger signal.
2. Description of the Related Art
In a video system, it is often required to generate a pulse signal having a predetermined time period width in response to a sync signal serving as a trigger. For example, it is often required to generate a pulse associated with automatic gain control (AGC) in the video system, i.e., a signal used to discriminate a pulse width of a horizontal sync signal.
In such a conventional video system, when a pulse signal having a predetermined time period width is generated in response to a sync signal serving as a trigger, a digital pulse generating circuit is used to count a predetermined number of clocks having a predetermined period and output a pulse. In such a pulse generating circuit, when a time period width of a pulse signal to be generated is set to be about twice that of the clock, an analog doubler is required, as shown in FIG. 6.
More specifically, a clock SC used as a system clock is converted into a clock having a frequency twice that of the clock SC by a doubler 20. An output from the doubler 20 is used as a clock of a counter 21, and the counter 21 receives a sync signal (SYNC) as a trigger input. Therefore, an output pulse having a predetermined time period width which is about twice that of the system clock SC can be obtained from the counter 21.
The doubler is often constituted by a pure analog circuit or a semi-analog digital circuit utilizing delay of a gate. In such a doubler, however, the number of parts is increased or a stable operation cannot be realized.
Thus, since the analog doubler used for the pulse generating circuit has the above drawbacks, in the entire conventional pulse generating circuit for generating a pulse signal having a predetermined time period width by counting a clock having a predetermined period, the number of parts is increased or a stable operation cannot be realized.